Stargazing allows observers to peer into the vastness of space. While a full moon drastically changes the experience, it does not completely prevent viewing the night sky. The sheer brightness of our closest celestial neighbor acts as a natural form of light pollution, impacting the visibility of fainter objects. However, astronomers and casual sky-watchers can still find rewarding sights on a moonlit night.
The Mechanism of Moonlight Interference
The full moon hinders deep-sky observation due to scattered light. The Moon reflects sunlight back toward Earth, which then scatters off gas molecules and aerosols in the atmosphere. This process creates a wide area of illumination, known as sky glow, which raises the background brightness of the night sky.
Sky glow washes out dim celestial targets like distant galaxies, nebulae, and faint star clusters. These deep-sky objects (DSOs) possess a low surface brightness, making them easily overwhelmed by the ambient moonlight. Even far from city light pollution, the full moon can make the night sky almost as bright as a suburban area.
The intense lunar light also negatively affects a stargazer’s dark adaptation, the process by which the eyes adjust to darkness. It takes the human eye about 20 minutes to achieve maximum sensitivity for detecting faint light. The continuous influx of bright moonlight interrupts this process, making it difficult to perceive the dim light from faraway stars and nebulae.
Celestial Targets That Withstand Bright Moonlight
Many celestial objects are luminous enough to overcome the sky glow. Planets remain excellent targets due to their proximity and high reflectivity. Bright examples like Jupiter, Saturn, Mars, and Venus are easily visible, even to the naked eye. Using a small telescope or binoculars can reveal details like Jupiter’s four largest Galilean moons.
The Moon itself is the primary target, though the full phase presents the Sun’s light hitting the surface straight on. This high-angle illumination reduces the shadows that typically highlight craters and mountains during other phases. Even so, a telescope allows for detailed observation of the lunar rays and bright features.
Bright open star clusters also hold up well against the increased background light. The Pleiades (M45) is a distinct grouping of stars that is bright enough to be seen, as are other dense groupings like the Hyades. Additionally, multiple star systems and double stars can be observed and resolved with a telescope due to their focused light.
Practical Strategies for Observing During a Full Moon
To maximize enjoyment when the Moon is full, employ specific equipment and timing strategies. When observing the Moon directly through a telescope, use a specialized lunar filter or a neutral density filter. These filters reduce the glare and prevent the intense light from compromising your night vision, enhancing the contrast of surface features.
Timing your observations can also make a difference, particularly by observing before or after the exact full moon date. The few nights surrounding the full moon are the brightest. Catching the Moon when it is slightly lower in the sky can reduce the scattered light.
Instead of hunting for faint deep-sky objects, concentrate your efforts on targets known to be bright. If using a telescope, aim for objects far from the Moon’s position to minimize scattered light. Binoculars are also very effective, offering a wider field of view for scanning brighter patches of the sky.